A stopper, often called a bung, is a device used to seal the opening of laboratory glassware in chemistry. Its primary purpose is to create a secure, physical barrier between the contents of a vessel and the external environment. This sealing action is fundamental to maintaining the integrity of chemical substances and preventing external contamination. The stopper is designed to fit snugly into the opening of flasks, test tubes, and other containers.
The Essential Function of Laboratory Stoppers
Stoppers serve multiple overlapping functions that are foundational to accurate and safe chemical work. The most immediate purpose is containment, which prevents the accidental escape of liquids or fine solid materials from the vessel. A tight seal also prevents the loss of volatile solvents or reagents through evaporation, ensuring the concentration of solutions remains constant throughout an experiment.
Beyond simple containment, stoppers maintain a controlled environment within the glassware. By forming an airtight seal, they prevent atmospheric contaminants, such as dust, moisture, or reactive gases like oxygen and carbon dioxide, from entering the container and interfering with the sample. This exclusion of external elements is especially important in moisture-sensitive or air-sensitive chemical reactions.
Stoppers also play a direct role in laboratory safety by sealing off hazardous materials. They safely contain volatile liquids, toxic fumes, or pressurized gases that might otherwise present a danger to personnel or the lab environment. When used in a multi-component apparatus, a stopper ensures a closed system, which is necessary for procedures like distillation or filtration under vacuum.
Common Types and Materials
Resilient materials, such as rubber, silicone, and cork, are widely used for general sealing applications due to their flexibility. Standard black or red rubber stoppers provide a good seal for aqueous solutions. Silicone stoppers are often preferred for high-temperature applications because of their superior heat resistance.
Specialized resilient stoppers, such as those made from polytetrafluoroethylene (PTFE) or neoprene, offer high chemical resistance to strong acids, bases, and organic solvents that would degrade standard rubber. In contrast, rigid materials like ground glass are used for precision work, forming a standardized, interchangeable seal known as a standard taper joint. These ground glass stoppers are chemically inert and provide an extremely precise fit, often used in volumetric flasks and reagent bottles.
Stoppers are also categorized by their structure, which dictates their function in an experimental setup. Solid stoppers are used for simple storage or sealing a vessel when no access is required. Single-hole and multi-hole stoppers are designed to accommodate thermometers, glass tubing, or stirring rods, allowing components to pass into the vessel while still maintaining a seal.
Rubber stoppers follow a standardized numerical sizing system, such as numbers 00 through 16, where the number corresponds to a specific tapered diameter to fit various laboratory glassware openings. Ground glass joints, however, are sized by a pair of numbers (e.g., 24/40), which refer to the diameter at the widest point and the length of the ground surface, both measured in millimeters.
Proper Selection and Safe Handling
The stopper must be chemically compatible with the substance it is sealing to prevent degradation of the material, which could lead to contamination or seal failure. For instance, a neoprene stopper should be chosen over natural rubber when working with petroleum products or certain organic solvents, which can cause natural rubber to swell.
Equally important is selecting the proper size to ensure a snug and reliable fit without being overly difficult to insert. A stopper that is too small will not seal effectively, while one that is too large may require excessive force, increasing the risk of cracking the glassware. When inserting glass tubing or a thermometer into a holed stopper, a thin layer of lubricant, such as glycerin or water, must be applied to the glass to ease its passage.
The proper technique for insertion involves protecting the hands with gloves or a cloth and using a gentle twisting motion rather than brute force to work the glass item through the hole. This prevents the glass from snapping and causing a severe laceration. For removal, the same twisting motion should be used to break the seal and avoid damaging either the stopper or the glass vessel.